How common is it to have an extra rib?

It's common. Extra ribs are more of a curiosity for the trainee in radiology than anything else! either there may be an extra rib at the top end or the bottom. Rarely, an extra cervical rib may press on some nerves or an artery and needs to be resected, but then the symptom is what brings the patient in to get the x-ray.
Extra rib. If you mean a cervical rib it's about 1 in 500 and much rarer to have a set of them.